The invention relates to a method for sending frames of data from at least two ingresses to one egress in an ATM network wherein each frame of data is partitioned into cells. Furthermore, the invention relates to an apparatus for sending as well as to an apparatus for receiving a frame of data in an ATM network wherein the frame of data is partitioned into cells.
ATM (asynchronous transfer mode) networks are well known and wide spread over the world. In an ATM network, the data to be transferred is put together in a frame which consists of a number of cells. The number of the cells depends on the amount of data to be transferred. Each of the cells includes 48 bytes data information and 5 bytes control information. The control information carries all necessary information for transferring the cell from its source to a desired destination. The control information includes a VPI (virtual path identifier) field and a VCI (virtual channel identifier) field. In particular the VPI field identifies the desired path of the data and is therefore an important feature of the respective connection.
The cells are sent from the source via a number of nodes to the destination. Each of these nodes between the source and the destination represents a source for its downstream node and is therefore called an ingress. As well, each of these nodes represents a destination for its upstream node and is therefore called an egress.
The connection between the source and the destination is called a point-to-point connection. There is only one path without any spreading or merging of other paths. As already mentioned, any such point-to-point connection is identified by its specific VPI field.
Another possibility is a point-to-multipoint connection. In this case, one source broadcasts the same data to a number of destinations. For that purpose, the ATM network provides a SETUP message and an ADD PARTY message. With the SETUP message, a point-to-point connection is established. Then, with the ADD PARTY message, it is possible to add further destinations to this point-to-point connection and to thereby build up the point-to-multipoint connection.
Each point-to-multipoint connection is identified by its specific VPI field. It includes one ingress which is followed by at least two egresses in order to spread the data to be transferred from the single source to the number of destinations. As there is only one source relating to one and the same VPI field, all cells arriving in the destinations and having an identical VPI field come from the same source. Cells from other sources can be seperated by their different VPI fields. Therefore, no problem arises in the destinations to combine the frame originally sent from the single source out of the received cells.
It is also possible to reverse the point-to-multipoint connection into a multipoint-to-point connection. For this purpose, a reverse tag may be included in the SETUP message and in the ADD PARTY message. Again, each specific multipoint-to-point connection is identified by its VPI field.
Such a multipoint-to-point connection includes at least two ingresses which are followed by only one egress. Thereby, the data to be transferred is merged together from the number of sources to the single destination. This has the consequence that cells from different sources may be interleaved when they are merged together. The cells arriving at the single destination are therefore mixed with respect to their different sources and have to be reassembled appropriately. As the VPI field of one and the same multipoint-to-point connection is identical for all cells sent from all sources, it is not possible to reassemble the cells of the different sources by the VPI field. Therefore, a mechanism is needed to reassemble the cells from the different sources correctly in the single destination.
A first mechanism is that each egress collects all cells that arrive on a specific input port and that have the same VPI field until the egress has collected all cells of one and the same frame. Then, these collected cells are forwarded to the next downstream egress continuously, i.e. no other cells from other ingresses are interleaved. Using this mechanism, the cells are not mixed and it is possible to reassemble the frames sent by the different sources as these frames arrive continuously at the single destination. However, this mechanism has the disadvantage that it introduces a significant delay in the transfer of the data.
In another mechanism, the VCI field of a cell is used to identify the particular source of the cell. Each source worldwide is assigned with a globally unique value which is then included by the source in the VCI field of each cell to be sent by the source. Due to the unique configuration, all cells arriving at a destination and having an identical value may then be reassembled into the same frame. This mechanism does not introduce a delay but it requires a globally unique assignment of values for the VCI field which is practically very difficult to guarantee.
It is therefore an object of the invention to provide a method and an apparatus for a multipoint-to-point connection to reassemble the cells from the different sources effectively in the single destination.
In a method as described above, the invention solves this object by the steps of: the egress creates an identification for each of the ingresses and sends the identification to the ingresses, each of the ingresses includes the received identification in each of the cells and sends the cells to the egress, and the egress combines the received cells into frames depending on the included identification.
As well, in an apparatus for sending as described above, the invention solves this object by means for receiving an identification, means for including the identification in each of the cells of the frame, and means for sending the cells.
As well, in an apparatus for receiving as described above, the invention solves this object by means for creating an identification, means for sending the identification, means for receiving cells, and means for combining cells into frames depending on the included identification.
In the mechanism according to the invention, an egress, e.g. the destination, assigns an identification to each of the ingresses, and each of the ingresses, e.g. all sources, include this identification in its cells. The egress then receives all transmitted cells with the included identification. Depending on the identification, the egress is able to reassemble the cells correctly into the frames of the different ingresses.
The mechanism according to the invention does not introduce any delay as the cells are transferred without any collection in the ingresses in between. As well, the mechanism according to the invention does not require any global configuration as the egress assigns an identification to each of the ingresses. The mechanism according to the invention only requires that during the establishment of the multipoint-to-point connection the respective identifications are transferred from the egress to the ingresses. However, this requires only very minor additional processing which happens prior to the transfer of the data and which therefore does not influence the transfer of the data as such. Furthermore, the mechanism according to the invention requires that all ingresses include the respective received identifications in the cells to be transmitted. However, this also requires only very minor additional processing during the provision of the cells with their control information in the respective egresses which also happens prior to the transfer of data and which therefore does also not influence the transfer of the data as such.
In an advantageous embodiment of the invention, the identification is included in the SETUP and/or the ADD PARTY message in order to be transferred from the egress to the ingresses. As described, these messages are necessary to build up a multipoint-to-point connection. Therefore, the transfer of the identification from the egress to the ingresses does not require any additional time. It is sufficient to include the identification in the respective message and to transfer the message as usual.
In another advantageous embodiment of the invention the identification is included in the VCI field of the cells when being transferred back from the ingresses to the egress. By this inclusion, all of the cells are identified such that the egress knows from which ingress the cells comes from. In other words, by including the identification in the VCI field of the cells, all cells are identified with respect to their particular ingress. As already described, the VCI field is part of the control information of all cells and is therefore necessary to build up a cell. Therefore, the transfer of the identification in the VCI field does not require any additional time. It is sufficient to include the identification in the VCI field of each cell and to transfer the cell as usual.
Another advantageous embodiments of the invention comprises the further step of providing a number of identifications to each of the ingresses and then each of the ingresses providing one of the identifications to a requesting further ingress. This procedure enables the adaption of the mechanism according to the invention to cases in which the multipoint-to-point connection is not initiated by the destination but by one of the nodes in between. As well, this procedure enables that another new node joins the multipoint-to-point connection in a later point in time.